Environmental concerns have motivated the implementation of emission requirements for internal combustion engines throughout much of the world. Catalytic converters have been used to eliminate many of the pollutants present in exhaust gas; however, a filter is often required to remove particulate matter, such as, for example, ash and soot. Wall-flow particulate filters, for example, are often used in engine systems to remove particulates from the exhaust gas. Such particulate filters may be made of a honeycomb-like substrate with parallel flow channels or cells separated by internal porous walls. Inlet and outlet ends of the flow channels may be selectively plugged, such as, for example, in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas. Particulate capture by the porous walls can occur in two different stages: at first, inside the porous wall (deep-bed filtration), and later, on the porous wall in the flow channels (cake-bed filtration).
In this manner, wall-flow particulate filters have been found to be effective in removing particulates, such as, for example, ash and soot, from exhaust gas. However, the pressure drop across the wall-flow particulate filter increases as the amount of particulates trapped in the porous walls and channels increases. For a filter that is not conditioned (e.g., that does not have enough of an ash layer to stop particulate matter from penetrating the porous filter walls), there is generally a rapid increase in pressure drop during the initial deep-bed filtration stage (typically less than 1 g/L (gram per liter)), followed by a gradual increase in pressure drop with particulate loading during the cake-bed filtration stage. The increasing pressure drop results in a gradual rise in back pressure against the engine, and a corresponding decrease in the performance of the engine. Accordingly, soot is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated.
One conventional regeneration technique, for example, involves heating the inlet exhaust gas to a temperature which supports burning of the soot off of the filter. This process, however, also releases energy in the form of heat, which raises the temperature of the filter. If the soot level in the filter becomes too high, regeneration can raise the filter's temperature to the point of failure, which may result in thermally induced cracking or melting of the filter. Filter failure can result in a severe reduction in the filter's filtration efficiency, requiring replacement. Accordingly, in order to avoid high filter temperatures that can lead to filter failure, it is advantageous to carry out regeneration at a sufficiently low soot level.
However, it also may be desirable to avoid triggering regeneration at too low a soot level, since to do so may result in an unduly large number of regeneration events. Repeated regeneration events can have negative impacts, such as, for example, by causing oil dilution, NOx penalty, fuel penalty, and/or engine power loss. It may therefore be desirable to permit a relatively large interval between regeneration events, while ensuring that regeneration is triggered prior to the soot level reaching a critical soot level to avoid regeneration at high filter temperatures.
The pressure drop, for example, across a wall-flow particulate filter can be used as a way to monitor the level of soot in the filter and trigger regeneration when the soot mass reaches a critical limit. In practice, however, there is some uncertainty as to the actual soot level in the filter due to such factors as measurement error, variations in pressure drop response from one filter to another, and/or the change in pressure drop response as a function of the presence of ash in the filter (e.g., whether a filter is new or has accumulated ash (ash conditioned)). In the case of a filter demonstrating a relatively low pressure drop response with respect to soot loading during the cake-bed filtration stage (e.g., the slope of the curve of pressure drop versus soot load level is shallow), this uncertainty can be rather large. Nevertheless, due to the objective of increasing fuel efficiency, some conventional approaches have concentrated on minimizing pressure drops across the filter regardless of particulate loading, thereby resulting in a low pressure drop response to soot loading not only during the initial deep-bed filtration stage, but also during the cake-bed filtration stage.
Maintaining a low pressure drop response, however, can also have a negative impact on fuel economy. More specifically, a low response of pressure drop to an increase in soot loading during the cake-bed filtration stage, especially in those cases where pressure drop is used to trigger a regeneration event, can negatively affect fuel efficiency. Regeneration requires energy input to the system to raise the temperature of the inlet gas to a sufficiently high level to initiate burning of the soot. This energy input typically comes from a post-injection of fuel, but whatever the energy input device, the result is a loss in fuel economy.
It may be desirable therefore to provide a filter that exhibits a relatively high pressure drop response to particulate matter loading (i.e., relatively steep slope for the curve of pressure drop versus soot load level) during the cake-bed filtration stage, while exhibiting a low pressure drop prior to particulate matter loading (e.g., when the filter is clean), and a relatively low pressure drop response to particulate matter loading (i.e., relatively shallow slope for the curve of pressure drop versus soot load level) during the deep-bed filtration stage. It also may be desirable to provide a filter that achieves a relatively high filtration efficiency (FE). Accordingly, it may be desirable to provide filters having geometric properties and microstructural properties that achieve the aforementioned desirable features.
It also may be desirable to provide a filter regeneration technique that does not result in triggering regeneration too early (resulting in too frequent regeneration), while ensuring that regeneration is performed prior to the filter reaching a critical particulate matter load level. In other words, it may be desirable to provide a filter and a regeneration technique that triggers regeneration at particulate matter load levels that are closer to critical load levels than those used by conventional techniques to trigger regeneration.